1. Field of the Invention
This invention relates to a method of manufacturing a rhenium coating/film, more particularly to, a method of manufacturing a rhenium coating/film that can withstand a highly corrosive and erosive environment at a high temperature.
2. Description of the Prior Art
Some space and missile propulsion systems must withstand conditions that disrupt the structural integrity of the material. The temperature of the combustion products may be as high as 2,000° to 3,000° C. Brittle materials tend to fail rapidly because of an inability to withstand both structural and thermal shock. Premature failure because of cracking under shock or thermochemical failure greatly limits the useful life and reliability of an item, e.g., a missile nozzle or a gun tube.
The carbon fiber reinforced carbon matrix composites, i.e., carbon—carbon composite, are a light weight, high strength, refractory, structural material whose strength increases with temperature, even at temperatures in excess of 2,000° C. Carbon and carbon—carbon structural elements can be used at temperatures where other refractory materials have lost their practical strength. Unfortunately, carbon is susceptible to oxidation and erosion when exposed to oxidizing environments, even at moderate temperatures. Carbon is particularly vulnerable to oxidation in hostile environments and thus requires a protective coating.
Rhenium is a refractory metal that has been shown to successfully protect materials from erosive and corrosive environments. Rhenium can provide erosion resistance for components in high temperature rocket engines and hot gas valves. Rhenium has many advantages over other candidate liner materials. Rhenium has a high melting point of 3180° C./5756° F. exceeding all other metals except tungsten. Rhenium has a ductile-to-brittle transition temperature well below room temperature. With approximately 35% ductility as measured in elongation, and a tensile strength dropping from >1172 MPa (<170 ksi) at room temperature to >48 MPa (<7 ksi) at 2700° C., rhenium is virtually inert to thermal shock. Rhenium does not form a carbide and yet it does have a significant solubility for carbon that ensures excellent bond strength between the materials.
One previously used structure was composed of metallurgically bonded layers of platinum group metals such as ductile iridium and a refractory metal such as ductile rhenium that possesses both the high temperature corrosion resistance and the high temperature structural strength for high stress, high temperature applications, see, U.S. Pat. No. 4,917,968 to Tuffias et al., titled High Temperature Corrosion Resistant Composite Structure that issued on Apr. 17, 1990.
Increasing the thickness of the structure was originally believed as the means to increase the strength of the structure, particularly the thickness of the refractory metal layer. Increasing the thickness also increased the weight, the heat capacity, the cost and the difficulty of manufacturing the structure. Chemical vapor deposition, CVD, procedures were used to form successive layers of a structure under conditions such that the layers are bonded together by an interlayer, which is an admixture of the two adjacent layers. The metallurigically bonded iridium-rhenium structure described in this patent was capable of withstanding structural and thermal shock and the very high temperatures except that increasing the thickness did produce the undesirable results.
The usual method for applying a coating of rhenium to a substrate is by chemical vapor deposition, CVD. Rhenium pellets are exposed to a stream of chlorine gas in the presence of hydrogen gas in a reaction chamber at a temperature between 500° and 1200° C.
U.S. Pat. No. 5,577,263 to West discloses a chemical vapor deposition method in which a stream of gaseous rhenium hexafluoride flows onto a carbon substrate in the presence of a stream of sufficient hydrogen to cause a reduction of the rhenium hexafluoride to rhenium metal having an average grain diameter between 0.1 and 25 micrometers.
U.S. Pat. No. 5,874,015, titled Method for Making a Rhenium Rocket Nozzle to Mettendorf et al., issued on Feb. 23, 1999, discloses a method of depositing a number of layers of rhenium on a molybdenum mandrel. First, a layer of rhenium is deposited by CDV over the nozzle portion of the molybdenum mandrel, then rhenium wire is wrapped around the rhenium layer and then another layer of rhenium is deposited using CVD. Alternating layers are applied until a desired thickness is obtained. Finally, the molybdenum is etched away leaving the rhenium nozzle.
U.S. Pat. No. 5,855,828, titled Method of Forming a Composite Structure such as a Rocket Combustion Chamber, issued to Tuffias et al. on Jan. 5, 1999, discloses a carbon—carbon composite having a roughened dendritic surface of rhenium formed by CVD.
U.S. Pat. No. 5,935,351 to Sherman et al. for Method for Making a High Temperature, High Pressure, Erosion and Corrosion Resistant Composite Structure and issuing on Aug. 10, 1999, discloses depositing a layer of rhenium on a substrate by CVD. The composite formed was a gun barrel and rhenium was the corrosion and erosion resistant inner layer.
CVD is an expensive method for applying a layer of rhenium to substrates and a difficult method to use to control the thickness of the layer. There is a need for a cost effective, lightweight composite combustion chamber that will possess the high temperature structural strength for high stress, and can withstand the high temperature corrosive and erosive environment that the combustion chamber is subjected to during a period of high temperature.
In addition to the cost consideration, the CVD process becomes more complicated for the deposition of rhenium inside a nozzle or gun tube. The CVD process produces a coating on the exposed surface. The CVD process also requires additional treatments to produce rhenium film of different thicknesses.